“It’s been assumed that the process of retrieving a memory is cued by an external stimulus,” says Charan Ranganath, professor of psychology at the University of California, Davis. “But we found that the levels of brain activity before items came up were correlated with memory.”

Researchers measured a particular frequency of brainwaves called theta oscillations in the brains of volunteers during a memory test. Theta waves are associated with a brain that is actively monitoring something. For example, rats show high theta waves while exploring a maze.

In the memory test, the volunteers had to memorize a series of words with a related context. They later had to recall whether they had seen the word previously and the context in which the word was seen.

High theta waves immediately before being prompted to remember an item were associated with better performance.

The work goes against the assumption that the brain is waiting to react to the external world, Ranganath says. In fact, most of the brain is busy with internal activity that is not related to the outside world—and when external stimuli come in, they interact with these spontaneous patterns of activity.

The next step, Ranganath says, is to investigate whether it’s possible to deliberately put the brain into a better state for memory recall.